We are investigating the structure of animal cell membranes, with particular emphasis on the dynamics of membrane components. We are using photoreactive and paramagnetic lipid analogues in conjunction with membrane systems that range from simple model vesicles to membranes which envelope animal cell viruses and eukaryotic cells. Recent work focusses on the disposition, organization, and dynamics of two classes of membrane proteins: those that are considered integreal membrane components and those that insert into or cross the membrane bilayer as a prelude to an assembly or transmembrane signalling event. The probes we synthesize and use are rigorously characterized because the more we know about the location and reactivity of a specific probe, the more we can discover about the membrane, for example, the kinetics of penetration and fate of newly inserted molecules. The two main problems under investigation are the lipid-protein and protein-protein interactions involved in complement-mediated lysis, including assembly of C5b-9, and the model of entry of cholera toxin, including the destination of functional A1 subunits. Both problems are bieng studied in model membrane systems designed to permit the monitoring of functional activities.